Epoxy structural adhesive glass transition temperature

Urethane structural adhesives have a morphology that is inverse to the toughened epoxy just described. The urethanes have a rubber continuous phase, with glass transition temperatures of approximately —50°C. This phase is referred to as the .soft segment . Often, a discontinuous plastic phase forms within the soft segment, and that plastic phase may even be partially crystalline. This is referred to as the hard segment . A representation of the morphology is shown in Fig. 3 [34]. 

The two-component urethane structural adhesives are among the most difficult to characterize, simply because of the widely varying properties that are possible. These adhesives may be rigid plastics similar in modulus to standard epoxy adhesives, with glass transition temperatures of the cured adhesive being approximately 60°C. 

Adhesive strength is evaluated at room temperature as well as at the extreme temperatures of —65°F and 180°F. Aircraft structure can reach —65°F at cruise altitudes and 180°F on the ground in a hot, sunny location. The types of toughened epoxies commonly used for metal bond adhesives have glass transition temperatures not much greater than 200°F, so properties fall off drastically at higher temperatures. 

BMI polymers have glass transition temperatures in excess of 260°C and continuous-use temperatures of 200-230°C. BMI polymers lend themselves to processing by the same techniques used for epoxy polymers. They are finding applications in high-performance structural composites and adhesives (e.g., for aircraft, aerospace, and defense applications) used at tem-peratrues beyond the 150-180°C range for the epoxies. Bisnadimide (BNI) polymers are similar materials based on bisnadimides instead of bismaleimides. 

The increase in the glass-transition temperature Tg of a film 0.01 X 10 m thick is associated with limitation of the mobility of the polymer chains near the solid surface. The character of Tg change for the surfaces of basalt-1 and basalt-2 is indicative of a complex structure of the boundary layer. For films (0.01-0.03) x 10 m thick on the log Yg-AjT curve, no fracture is observed in the investigated temperature range. We assume that a bormdary layer of epoxy composition 0.03 X 10 m thick does not change the polymer condition. The high-energy surface can selectively sorb the epoxy resin, as a result of which the adhesive layer is improved by hardener, and the stoichiometry of the composition becomes disturbed. The polymer layer emiched in... 

The durability of epoxy-aluminium joints that used a homopolymerised epoxy resin was studied by researchers based in Spain [15], and the effects of relative humidity, temperature, and salt concentration analysed. The homopolymerised epoxy resin absorbed little water (1.5 wt%) because of its non-polar network structure. Increasing relative humidity and temperature enhanced water uptake, but the joint strength remained constant because of epoxy plasticisation. A saline environment was damaging to the adhesive joints because of metal corrosion, but was not significantly harmful to the epoxy resin because of the lower diffusion coefficient of salt water. The decrease in glass transition temperature of the epoxy adhesive due to water absorption was dependent upon only the amount of absorbed water and was independent of hydrothermal ageing conditions. The durability of epoxy adhesive joints made underwater has been studied [16]. 

---